1. Field of the Invention
The present invention relates to an optical scanning apparatus used for an electrophotographic type image recording apparatus.
2. Description of the Related Art
There has been known an electrophotographic type image recording apparatus for receiving image information from a computer and forming an image on an image receiving medium in accordance with the image information. The image recording apparatus is equipped with an optical scanner which receives the image information from the computer and modulates and deflects laser beam emitted from a laser source in accordance with the image information.
FIG. 1 shows a conventional optical scanner 31 employed in the electrophotographic type image recording apparatus. The optical scanner 31 receives the image information from the computer and optically scans a cylindrically-shaped photosensitive drum 33 in raster pattern to form a latent image thereon. As shown in FIG. 1, the optical scanner 31 is positioned to confront the photosensitive drum 33. All the components of the optical scanner 31 are encased in a casing 32. In the casing is installed a semiconductor laser 34 which faces a polygonal mirror 40. The polygonal mirror 40 is axially fixed to a rotating shaft of a constant-speed rotating motor 42. Installed between and justified with the semiconductor laser 34 and the polygonal mirror 40 are a collimating lens 35 and a cylindrical lens 36. Installed between the polygonal mirror 40 and the photosensitive drum 33 are a series of imaging lenses 43 including an f.theta. lens.
The laser 34 is electrically connected to a computer device and receives image signals from the computer. The laser 34 emits laser beam modulated in accordance with the image signals. The laser beam emitted from the laser 34 passes through the collimating lens 35 where it is converted into a parallel beam. The parallel beam then passes through the cylindrical lens 36 where it is focused to strike a surface of the polygon mirror 40 in an elliptic beam spot. Since the polygonal mirror 40 is rotated with a constant speed by the motor 42, the laser beam reflects off the polygon mirror 40 to be deflected therefrom at a constant angular velocity. The laser beam then passes through the imaging lenses 43 and strikes a peripheral surface of the photosensitive drum 33 where an electrostatic latent image is produced. The latent image is then developed through a well-known electrophotographic process and is transferred to be fixed on a hard copy medium such as a paper.
The above-described conventional optical scanner has a problem, however, in that the polygonal mirror 40 should be equiangular, which requires high-precision production, to prevent irregularities in spacings between scanning lines that will possibly appear if surfaces thereof are at varying angles with respect to the axis of the polygonal mirror 40.
Inclusion of the cylindrical lens 36 and the imaging lenses 43 is a well-known technique for reducing such irregularities. However, this technique creates additional problems wherein the number of parts and the complexity of the apparatus increase. It is noted that the imaging lenses 43 include a plurality of f.theta. lenses each of which has a complex shape. The increase in the number of the parts further requires an intricate positioning operation of the parts so that all the parts may be axially aligned with respect to an optical axis of the laser beam. More specifically to say, the semiconductor laser and the collimating lens should be positioned relative to each other so that the laser beam from the semiconductor laser may be reliably converted into the parallel beam. The cylindrical lens and the imaging lenses should be positioned relative to the polygon mirror so that the laser beam may be focused on the polygon mirror and may be focused on the photosensitive drum in a beam spot having a desired spot shape.
Furthermore, the polygon mirror should be symmetrical with respect to its axis so that it may be mounted on the motor rotating shaft in a balanced manner, to thereby prevent variations in the rotation speed of the motor. Accordingly, the polygon mirror should be produced with high accuracy.
High precision is also required for attaching the polygonal mirror 40 to the motor 42. If the two are not precisely axially aligned, the large mass and subsequent inertia of the polygonal mirror 40 cause large variations in rotation speed of the motor 43.
As described above, each of the parts employed in the conventional optical scanner has a complex shape. The number of the parts employed in the conventional optical scanner is large, and therefore positioning operations for the parts becomes intricate. Accordingly, the conventional optical scanner is expensive.